Water Hammer – Ask an Expert

When things go ‘bump’ in the night, or even in the day, it can be disconcerting if you don’t know what’s causing it.  One R&B customer has experienced this in the form of a phenomenon in her plumbing called water hammer.  Here is her letter to our R&B Ask an Expert Series.   Her letter, might be short but it’s direct:

Dear Rob,

How do I fix an issue with my pipes knocking when I run my washer?


Knock, knock, Go away!

Dear KKGA,

If you don’t have master plumber and R&B expert Rob Blaksic to ask, ‘Banging in the pipes when the washer is on’, are actually great key words to find the answer to this question.  Don’t go to Google though, you have a local expert right here!

What is it?

Banging in the pipes can be caused by a few different things, but the most common is something called ‘water hammer’.  

What Causes Water Hammer?

When water is turned on and off suddenly, the inertia that has built up exerts its energy on the pipes that surround it, causing them to physically move in the wall.  That’s the bang or ‘hammer’ sound that is heard.   One of the most common place that this happens in washing machines during the rinse cycle as water is added to a load.   Water hammer happens more in houses with high water pressure.   

What will make it stop

Checking the house pressure reducing valve (PRV) or water pressure, and then installing or replacing the hammer arrestors on some key appliances will do the trick. 

Water arrestors by their design are an addition to the pipe.  A little air chamber that extends perpendicular off each that is sealed off from water.  When the water moves suddenly in the pipe that air chamber absorbs the shock.  As they get used though,  over time they wear out  making them ineffective.  

The bad news

Letting it go and leaving it untreated is a bad idea.  The movement of the pipes will increase and the wear and friction on the pipes will lead to leaks. Not to mention, it’s noisy.

The good news

Water hammer is not expensive to repair. The arrestors themselves are usually in the $50 range and it’s a basic service call to put them in.  The house PRV will have to be checked, but overall, it’s less expensive than most plumbing repairs and better for your appliances as well.  

Consider putting them on your dishwasher and on your clothes washer for sure, but also any quick closing sudden on / off solenoid valve too.  

Why it suddenly started or what makes it worse 

Your pressure reducing valve (PRV) is installed on the incoming water line of every home and reduces the psi of the city pressure down to a household pressure.  This little devil is also a ‘wear item’ and as they begin to age over time the water pressure in the home will begin to increase, causing the water hammer effect to either start or become worse.  It can also cause homes with no water arrestors and no issues in the past, to suddenly have issues.  If this sounds like your experience, then you should probably start with a test of your PRV.

In Summary

Banging of the pipes is usually a very simple fix and one that should be addressed.  Remember to also ask your favorite plumber to look at the potential for underlying causes such as from the PRV.